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TEXTBOOK OF PLANT PHYSIOLOGY 



and finally the sac will absorb all of the iron from the outer 

 solution. 



As protoplasm is rather permeable to substances dissolved in 

 water, they might be washed out from the cells. This leaching, 

 however, is prevented due to the fact that the reserve substances 

 are either insoluble in water, as starch and oil, or are in a colloidal 

 state, as protein, and inulin. When seeds germinate or when 



organs containing food reserves are emptied, 

 these substances return to their former 

 soluble and mobile condition. They leave 

 the cells in which they had accumulated, 

 moving towards the growing organs. 



This general mechanism of the absorption 

 and accumulation of substances in the cell, 

 however, does not always hold true. There 

 are cases when soluble substances accumu- 

 late in great quantity, and remain in the 

 Fig. 46.— Colloidal sac same state in which they were when entering 

 with tannin placed in the cell Thus, for instance, in the bulb of 



solution of an iron salt. . . . , , 



the common onion are stored considerable 

 amounts of glucose. In pigweed and other plants much potas- 

 sium nitrate accumulates. It must be noted that the concentra- 

 tion of nitrates in the root cells exceeds their concentration in 

 the soil solution. Attempts have been made to explain this 

 occurrence by the fact that sugar and potassium nitrates are 

 present in these cells in special unstable combinations with 

 some substances of the cell sap. All efforts of isolating such 

 combinations have failed, however. 



At the present time, the explanation of these phenomena is 

 sought in the so-called membrane equilibrium of Donnan. 



If a membranous sac impermeable to colloids containing a 

 readily ionizable salt, one of whose ions is of a colloidal character, 

 such as sodium proteinate, a sodium salt in which the role of an 

 acid is played by some protein, is immersed in water, the sodium 

 ions, though able to pass through the membrane, will not diffuse 

 out of the sac, being retained by the electrostatic attraction of 

 anions and cations. If some easily penetrating salt is added to the 

 water, for instance, sodium chloride, then in the process of osmosis 

 the Na and CI ions will diffuse through the septum and finally 

 there will be established an equilibrium in which products of the 



